Photographic process



Patented Nov. 21, 1967 3,353,957 PHOTOGRAPHIC PROCESS Ralph Kingsley Blake, Westfield, N..l., assiguor to E. I. du Pont de Nemours and Company, Wilmington, Del.,

a corporation of Delaware No Drawing. Filed Oct. 12, 1964, Ser. No. 403,329 8 Claims. (CI. 96-29) ABSTRACT OF THE DISCLOSURE Process for forming positive images comprising:

(1) Exposing an outer silver halide layer -of a photographic element having a contiguous inner silver halide layer,

(2) Developing the outer layer with a solution having silver diffusion transfer characteristics containing a hydroxybenzene compound, a different developing agent and a silver halide solvent, and

(3) Removing the outer layer.

is made by exposing a high-speed, large grain gelatinosilver halide emulsion layer, usually on a transparent support to an original or image by means of a camera or other exposing means. The exposed layer is developed, fixed, washed and dried. A positive imagerepresenting the original scene is then obtained by exposing, usually, a slower speed small grain gelatino-silver halide emulsion layer coated on a paper support by a contact printer or with a projection enlarger to the above negative. The exposed element is then developed, fixed, washed and dried.

All of the above operations are obviously costly and time consuming. A newer process involves either one or two elements, one'of which is a high-speed negative silver halide emulsion and the other is a so-called image receptive layer. In this process, the negative element is exposed in a camera in the same manner as explained above; 'but the element is processed in a special type of developer which contains a silver halide solvent. During development, the exposed silver halide corresponding to the negative'is reduced to metallic silver; but at the same time, the unexposed and undeveloped silver halide migrates to the image-receptive layer, either by virtue of the fact that the negative emulsion was coated over the image receptive layer to form an integral product, or because the two elements, the image-receptive layer and the negative emulsion layer coated on separate supports are held in intimate contact during the above development. The image-receptive layers may be of several different types but generally comprise particles dispersed in a colloid car- ,rier, e.g., gelatin, of substances capable of catalyzing the reduction of silver halide complexes to silver. It is clear that several advantages accrue from this type of processing. Only one exposure step and one development step is High speed taking emulsions are generally large grained and have low developed-silver covering power. This, of

course, results in lower maximum density and lower contrast in the final negative image.

soj

It is an object of this invention to provide a new process of silver transfer development. It is a further object to provide a process of silver transfer development which yields images having higher maximum densities and higher contrasts. A still further object is to provide a silver transfer process which utilizes large grain, high speed, silver halide emulsions to produce final images of developed silver having a high covering power. A further object is to provide a silver transfer process which affords a highly flexible method of controlling speed, contrast and density in photographic images. A still further object is to provide a silver transfer process which is simple to perform and may be applied in practically all fields where conventional negative/positive, inverse transfer, reversal, reflex copy and other photographic processes are normally used.

A process of the invention, in one of its aspects, comprises:

(1) Exposing, imagewise, the outer light-sensitive silver halide emulsion layer of a photographic element having a contiguous inner, non-fogged silver halide layer on a support, the silver salt in said inner layer being at least as soluble aqueous developer solution as the silver salt in the outer layer,

(2) Developing the outer layer with an aqueous developer solution having silver diffusion transfer characteristics containing:

(i) a developing agent taken from the group consisting of a polyhydroxybenzene, p-aminophenol and 2,4-diaminophenol.

(ii) at least one developing agent taken from the group consisting of a polyhydroxybenzene, p-arninophenol, 2,4-diaminophenol, p-methylaminopheno], l-phenyl-3-pyrazolidone, 1-phenyl-4-methyl-3- pyrazolidone, and ascorbic acid, and

(iii) a silver halide solvent, and, if desired,

(3) Removing the outer layer and, if desired,

(4) Reducing any remaining positive silver salt image.

In a further aspect of the invention, the process comprises:

(a) Exposing, imagewise, the outer light-sensitive silver 7 halide emulsion layer of a photographic element having a contiguous inner developable silver halide layer on a support, the silver salt in said inner layer being at least as soluble as the silver salt in the outer layer,

(b) Developing said exposed outer layer to form a silver image in said layer, by a developer composition sub stantially devoid of silver diffusion transfer characteristics;

(c) Developing the outer layer with an aqueous developer solution having silver diffusion transfer characteristics containing:

(i) a developing agent taken from the group consisting of a polyhydroxybenzene, p-aminophenol and 2,4-diaminophenol,

(ii) at least one developing agent taken from th group consisting of a polyhydroxybenzene, p-aminophenol,- 2,4-diaminophenol, p-methylaminophenol, l-phenyl-3-pyrazolidone, 1-phenyl-4-methyl-3- pyrazolidone, and ascorbic acid, and

(iii) a silver halide solvent, and, if desired,

(d) Removing the outer layer, and, if desired,

, (e) Reducing any remaining positive silver salt.

According to a preferred aspect of this invention, the

I outer silver halide layer is a'high-speed, large grain, low

covering power silver halide emulsion layer containing at least80 grams of gelatin per 1.5 moles of silver halide 'and the inner silver halide emulsion layer is a hardened emulsion containing 15-80 grams, and preferably 30-60 grams of gelatin per 1.5 moles of silver halide.

In the process just described, the development to form the negative image can be completed prior to the development in the presence of the silver halide solvent or the latter can take place during or subsequent to the development to form a negative image.

The outer emulsion layer (1) can, for example, be a sensitized, gelatino-silver bromoiodide emulsion layer and the inner layer a hardened, low-speed, high covering power gelatino-silver chlorobromide emulsion layer containing not more than 80 grams of gelatin per 1.5 moles of silver halide (II) on a film, paper or other support.

Suitable silver halide solvents include sodium, potassium and ammonium thiosulfates.

With a structure of the above type, silver transfer development may be carried out in a single step to form a silver image in (I) and a silver salt image in (II). This is then followed by washing in warm water to remove layer (I). It is essential in the single development process that the silver salt in layer (II) must be at least as soluble as the silver salt in layer (I). The positive silver salt image formed in layer (II) may be made visible by various means, the preferred of which is development by a fogging developer which reduces the positive silver salt image to a visible silver image. The image may also be made visible by toning or fogging, followed by conventional black and white or color coupling development.

The process may also be carried out by developing with a low solvent developer first, followed by a second silver salt transfer developer and warm water wash off to give a positive silver salt image which may be reduced to a visible silver image as described above.

Processing may be carried out in a tray, tank or any other conventional equipment well-known in the art.

In general, for the elements constructed as described above, the best results will be obtained with fast acting, high energy, high pH developers containing ph'enidone,

hydroquinone and .a silver halide solvent such as sodium thiosulfate. In addition it is desirable to include either in the developer or in the top emulsion layer (I) a substance which inhibits tanning due to oxidized developer. There are many such compounds but mercapto compounds such as l-phenyl-S-mercapto tetrazole have been found suitable. Compounds such as urea and resorcinol which act as gelatin solubilizers are also suitable. However, the type of developer will depend on the type of emulsions used and the results to be obtained. For example, under certain conditions, either a positive or a negative tanned image may be obtained with a one step silver transfer development.

In carrying out the process, one can use an element wherein both silver salt emulsion layers are hardened. In this case, of course, the high degree of adhesion between the two layers would make the removal of layer (1) diflicult if not impossible. Separation, however, may be accomplished by placing a thin, soft, water difiusible layer between layers (I) and (II).

The invention will now be illustrated, but is not intended to be limited, by the following examples.

EXAMPLE I A fine grain gelatino-silver chlorobromide emulsion containing 70 mole percent chloride and-3O mole percent bromide was prepared-in a conventional manner "by precipitating the silver halide in an aqueous gelatin medium by reacting soluble silver nitrate and soluble halide salts. The resulting emulsion which contained about 15 grams of gelatin per 1.5 moles of silver halide was freed or unwanted soluble salts formed by the precipitation reaction by washing by coagulation in the manner taught by Waller et al. U.S. 2,489,341. The emulsion was redispersed and without digestion or the addition of sensitizers but with the addition of a hardener and 30 g. of gelatin, it was coated :on a photographic film support of the 'type described in Example IV of Alles, U.S. Patent 2,779,684, on which had been coated a thin anchoring substratum layer of gelatin (0.5 mg./dm. The emulsion was coated to give a coating weight of about rng./dm. of silver bromide equivalent. Over this layer there was coated at about 50 mg./drn. of silver bromide equivalent an emulsion similar to the above containing about 80 g. of gelatin per 1.5 moles of silver halide, except that the emulsion was digested to bring it up to normal speed and sensitized with an ortho optical sensitizing dye of the type disclosed in Brooker et al. U.S. Patent 2,493,748. Before coating, there was also added to the emulsion .10 gram of 1- phenyl-S-mercaptotetrazole per 1.5 mole of silver halide for the purpose of decreasing the solubility of the silver halide in the outer layer.

The coated emulsion was then dried in a conventional manner. The resulting element was given a 10-second exposure through a neutral density step wedge in which the density of each step increased by the fourth root of two using a Type I-B intensity scale sensitometer equipped with a No. 2 sensitometer lamp at a scale setting of 81, a Wratten No. 79 filter at the shutter, and a white K-2 plate between the step wedge and the light source. The lamp was operated at an amperage of 8.5. The exposed element was developed under safelights for ortho sensitized film for one minute at 68 F. in the following developer.

Grams Hydroquinone 16.0 1-phenyl-4-methyl-3-pyrazolidone 1.0 Sodium sulfite (anhydrous) 80.0 Boric acid 5.5 Potassium bromide 2.0 6-nitrobenzimidazole nitrate 0.4 1-phenyl-5-mercaptotetrazole 0.1

Sodium hydroxide 24.0 Water to make 1 liter.

The element was then developed at 68 F. for two min- 'utes in a developer having the following composition.

Water to make 1 liter.

The element was then immersed in a short stop solution at 68 F. of the following formula:

Ml. Glacial acetic acid 20.10 Potassium bromide 3M 1.0.0.0

Water to make 1 liter.

The room lights were turned on and the element was washed in water at about F. for A minute to remove the top emulsion layer. The element was then developed at 68 F. for /2 minute in the first developer above and 'washed in cold water and dried. The densities, read in a conventional densitometer were as follows:

Exposure steps E13 v V 3.97

The positive characteristic curve plotted from the above data has a gamma of 18.0. A conventional lithographic developer having the formula:

. I film using the above type emulsion in a single coating and exposed and processed in the normal manner would have a gamma of only about 6 to 14.

An even higher gamma is obtained by processing the above element in the following manner.

A strip of the above element was exposed as described above and developed under safelight conditions for one minute in a developer at 68 F. having the following formula:

A sensitometric curve plotted from the above densities gave a gamma of 27.0. The gamma of high contrast lithographic films usually is in the range of 6 to 14, thus it can be seen that the process of the invention allows great flexibility in the selection of contrast.

A sheet of the film was exposed through a /2 neutral density wedge in a normal manner and then through the same wedge in crossed position to a -watt lamp at Ml. Glacial acetic acid Potassium bromide 3M 100 Potassium ferricyanide H O, w./v.) 100 Water to make 1 liter.

This treatment served tov bleach the toe in th top negative image. The film was washed for A minute and then developed at 68 F. for 2% minutes in a silver transfer Grams Hydroquinone 16.0 1-phenyl-4-methy1-3-pyrazolidone 1.0 Boric acid 5.5 Sodium sulfite (anhydrous) 80.0 Potassium bromide 2.0 Sodium thiosulfate (anhydrous) 30.0 6-nitrobenzimidazole nitrate 0.4 1-phenyl-5 -merc a p totetrazole 0. 1 Sodium hydroxide 24,0 .Resorcinol 16.0

Water to make 1 liter. i

Exposure steps E7; 4.0o+ E8 1.80 E9 .96 E10 p .05

( 10 volts for A minute. It was then developed for 2 Hydroquinone ffig minutes at 68 F. in a developer of Formula III and 2 1-pheny1-4-methy1-3..pyrazo1idone 1.0 mi s at 68 F. In a eveloper of Formula IV both Boricacid 1 5 5 of which are set forth above. The developed film was Sodium sulfite (anhydrous) 80.0 15 then bathed at 68 F. in the short stop solution, flashed Potassium bromide 2.0 to room light and washed in water at about F. to

6-nitrobenzimidazole nitrate 04 remove the top emulsion layer. The film was then deg gig giggggg ?f fi'f i? :III::"'::" veloped /2 minute at 68 F. in the developer for Formula Resorcinol 16.0 20 III, washed in water and dried. The densities of the crossed Water to make 1 liter. wedge at step 8 are shown in the following table.

Exposure steps E12 E13 E14 E15 E16 E17 E18 E19 E20 E21 wed e Unerossed 3.90 3.60 a. 10 2. is 1.70

Crossed Wedge Step8 2.72 2.44 1.92 1.21 .87 .90 .26 .06

It will be seen from the above data that the threshold shoulder speed at a density of about 2.70 is increased by a factor of about 11.3 or by about 3 /2 camera stops by the uniform minute white light exposure at step 8 on the crossed wedge.

EXAMPLE II A fine grain gelatino-silver chlorobromide emulsion containing 30 mole percent'bromide and 70 mole percent chloride was prepared in a conventional manner by precipitating the silver halides in an aqueous gelatin medium by reacting soluble silver nitrate and the proper soluble salts. The resulting emulsion was freed of unwanted soluble salts formed in the precipitation'reaction by washing by coagulation in the manner taught by Waller et al. U.S.

' ing 1.6 mole percent iodide and 98.4 mole percent bro mide. The emulsion was made in the conventional manner and-contained l50 grams ofgelatin per 1.5 molesofsilver halide and was sensitized by digesting with an organic sulfor compound and gold chloride, all according to procedures well known to those skilled in the art.

The coated film was dried and exposed for .08 second in the sensitometer of- Example I equipped with a V2 65' step wedge, a Corning filter 5850 plus a 0.3 neutralidensity filter with a 500-watt lamp having a color temperature of 2700 K. The film was developed for two minutes at 68 F. in a developer of Formula IV of Example I. The film was then immersed in the short stop bath of Example I for /2 minute and washed at about 125 F. to remove the unhardened silver bromoiodide negative layer. The film was developed in white light at 68 F. for 1 minute in a developer of Formula I of Example 1, washed and dried. The densities of the final positive image are given inthe following table.

A film support was coated with the hardened gelatinsilver .chlorobromide emulsion of Example I and then overcoated with an unhardened gelatino silver bromoiodide negative type emulsion which had been optically sensitized with apanchromatic sensitizing dye.

The coated film was given an exposure in a I-B negative sensitometer using a /2 step wedge which gave a .049 MCS exposure at step E11. The exposed film was processed in the same manner as Example II to give the following data in the final image.

Exposure steps E1 1.27 E2 1.22 E3 1.00 E4 .80 E5 .65 E6 .46

E7 .32 E8 .11 E9 :06 E10 .06

'It will be seen 'fromt'he above .data that a high effecfive speed-can be obtained in an .unsensitized fine grain silver chlorobromide emulsion.

EXAMPLE IV The film element of Example I was processed in the manner .described in :Example 11 to give the following results.

Exposure steps :In the :above examples -:the final positive images .are :formed by white light fogging and development of the :remaining'silver halide in the lower emulsion layer. However the use of .white lights is notnecessary. The Processes of the Examples can be carried .out completely :under safelight :conditions by adding 2.5 grams of potassium iodide :perliter to the developer of Formula I of Example I. This serves fthepurpose of fogging the remaining silver "halide grains and the results are similar to those obtained 'by' light fogging.

EXAMPLE V A hardened "fine grain gelatino-silver chlorobromide emulsion of the unsensitized type described in Example I but containing only grams of gelatin per 1.5 moles of silver halide was coated onzthe film support of that example to give a coating weight of about 1 16 mgJdm.

based on the weight of the silver halide. Over this layer there was coated at a coating Weight of 79 mg./dm. based on the weight of silver halide, a large grain, fully sensitized emulsion of the type described in Example II.

The coated film was dried and exposed and processed .as described in Example 11 except that the first developer contained 11.7 grams of resorcinol and 200 grams of sodium thiosulfate. The following results in terms of densities of the final positive image were obtained as shown in the following table.

Exposure steps E12 2.55 E13 2.55 E14 2.44 E15 2.13 E16 1.61 E17 1.30 E18 1.01 E19 .47 E20 .23 E21 .09

EXAMPLE VI A film element constructed as described in Example II and exposed as described in Example II was developed for 30 seconds in developer I of Example I and having the following composition:

Grams p-Hydroxyphenylaminoacetic acid (Glycin) 10 Boric acid 5.5 Potassium bromide 2.0 Sodium thiosulfate 15.0 Sodium sulfite 80.0 -nitrobenzimidazole nitrate 0.4 1;phenyl-5-mercaptotetrazole 0.1 Sodium hydroxide to adjust pH to 11.7 .Resorcinol 25 Water to make '1 liter.

Subsequent processing steps were the same as in vExample II .to give a final ,positive image having densities very similar to .the results in Example II wherein the maximumdensity was 3.8 and :the minimum density was .09 and the gamma was 3.8. When the exposed strip was processed for 16 minutes in the above developer alone, a positive image was obtained having a maximum density of 3.2, a minimum density of .10 and a gamma of 1.7.

EXAMPLE VII Example "VI was repeated except that 10 grams of p-aminophenolwas'used in place of the p-hydroxyphenyl- .aminoacetic acid'in the second developer and the pH was adjusted to "12.1. *Seusitometric results showed that a highly useful positive image was obtained having a maximum density of 3;8, a minimum density of 0.10 and a gamma of 3.1. Processing of the exposed strip for 12 minutes only 'in asingle developer containing the p-aminophenol and the sodium thiosulfate gave a positive image having a maximum .density .of 4.0+, a minimum density of 0.09 and a gamma of 3.0.

EXAMPLE VIII Example VI was repeated except that 6 grams of 2,4- diaminophenol was used in place of the p-hydroxyphenylaminoacetic acid and the pH .was adjusted to 7.3. The exposed strip was developed for 30 seconds in developer I of Example I and then for 10 minutes inthesolvent developer to give a good positive image having a maximum density of 2.1, a minimum density of 0.06 and a gamma of 1.0. Developing the strip 13 minutes in the single solvent developer gave a positive image having a maximum density of 3.2, .aminimurn density of 0.06-and a gammaof 1.3.

9 EXAMPLE 1x Example VI was repeated except that 4 grams of ascorbic acid was used in place of the p-hydroxyphenylaminoacetic acid and the pH was adjusted to 11.7. The exposed strip was developed for 1 minute in developer I of Example I and 12 mniutes in the above solvent developer containing ascorbic acid to give a positive image I having a maximum density of 3.5+, a minimum density exposed and non-developed areas on top of the positive silver halide image in the under layer. Upon further processing the hydrogen peroxide bathed element as described in Example II, the residual layer in the top layer served to reinforce the positive image in the bottom layer.

EXAMPLE X Example VI was repeated except that 6 grams of 2,4- aminophenyl and 2 grams of p-methylaminophenol were used in place of p-h'ydroxyphenylaminoacetic acid and the pH was adjusted to 7.95. Processing the exposed strip for 30 seconds in developer I of Example I and 10 minutes in the above solvent developer gave a positive image having a maximum density of 3.8+, a minimum density of .08 and a gamma of 1.7. Development of the exposed strip for 10 minutes in the solvent developer alone gave a positive image having a maximum density of 3.4, a minimum density of .08 and a gamma of 2.5+.

The novel process of this invention provides a rapid and simple means by which silver images can be made at the highest exposure speeds of conventional negative emulsions and yet have developed silver covering powers equal to or better than the slowest fine grain conventional emulsions. This characteristic may be used to give either (1) higher speed, contrast and maximum density or (2) equivalent sensitomertic characteristics with less silver halide coating Weight. Another advantage is that large changes in sensitometric characteristics are easily obtained through simple changes in processing.

The process of the invention provides a novel and practical means of making direct positive products having excellent sensitometric characteristics and ranging in speed from the slowest to the fastest conventional silver halide emulsions presently available. The process is particularly suited to those applications typical of recording films and papers which require maximum speed and contrast. Direct positive, high contrast graphic arts lithographic films of greatly increased speed as compared to conventional lithographic films are possible with the process of this invention. The novel process of the invention may also be utilized in the field of radiology, particularly in the medical X-ray field where improved sensitometric characteristics and speed are greatly desired.

I claim:

1. A process which comprises:

{1) exposing, imagewise, the outer light-sensitive silver halide emulsion layer of a photographic element having a contiguous inner, non-fogged silver halide layer on a support, the silver salt in said inner layer 'being at least as soluble in aqueous developer solution as the silver salt in the outer layer,

(2) developing the outer layer with an aqueous developer solution having silver diffusion transfer characteristics containing:

(i) a developing agent taken from the group consisting of a polyhydroxybenzene, p-aminophenol and 2,4-diaminophenol,

(ii) at least one developing agent taken from the group consisting of a polyhydroxybenzene, paminophenol, 2,4-diaminophenol, p-methylaminophenol, 1-phenyl-3 pyrazolidone, 1-pheny1-4- methyl-B-pyrazolidone, and ascorbic acid, and

(iii) a silver halide solvent, and

(3) removing the outer layer.

2. A process according to claim 1 wherein the emulsion layers are gelatino-silver halide emulsion layers.

3. A process according to claim 1 wherein the outer emulsion layer is a gelatino-bromoiodide layer and the inner layer is a gelatino-brornochloride layer predominating in silver chloride.

4. A process according to claim 1 embodying the additional step (4) reducing any remaining silver salt image.

5. A process which comprises:

(a) exposing, imagewise, the outer light-sensitive silver halide emulsion layer of a photographic element having a contiguous inner, non-fogged silver halide layer on a support, the silver salt in said inner layer being at least as soluble in aqueous developer solution as the silver salt in the outer layer,

(b) developing said exposed outer layer to form a silver image in said layer, by a developer composition substantially devoid of silver diffusion transfer characteristics;

(c) developing the outer layer with an aqueous developer solution having silver diffusion characteristics containing:

(i) a developing agent taken from the group consisting of a polyhydroxybenzene, p-aminophenol and 2,4diaminophenol,

(ii) at least one developing agent taken from the group consisting of a polyhydroxybenzene, paminophenol, 2,4-diaminophenol, p-methylaminophenol, l-phenyl-B-pyrazolidone, 1-phenyl-4- methyl-3-pyraz-olidone, and ascorbic acid, and

(iii) a silver halide solvent, and

(d) removing the outer layer.

6. A process according to claim 5 wherein the emulsion layers are gelatino-silver halide emulsion layers.

7. A process according to claim 5 wherein the outer emulsion layer is a gelatino-bromoiodide layer and the inner layer is a gelatino-bromochloride layer predominating in silver chloride.

8. A process according to claim 5 embodying the additional step (e) reducing any remaining positive silver salt image.

References Cited UNITED STATES PATENTS 2,565,376 8/1951 Land 96-29 2,647,056 7/1953 Land 96 29 2,937,945 5/1960 Weyde 96-68 X 3,050,391 8/1962 Thompson 9668 X 3,140,179 7/1964 Russell 96 6s 3,146,104 8/1964 Yackel 9668 X 3,232,759 2/1966 White 96 29 NORMAN G. T O'RCHIN, Primary Examiner.

MART s an Exa iner, 

5. A PROCESS WHICH COMPRISES: (A) EXPOSING, IMAGEWISE, THE OUTER LIGHT-SENSITIVE SILVER HALIDE EMULSION LAYER OF A PHOTOGRRAPHIC ELEMENT HAVING A CONTIGUOUS INNER NON-FOGGED SILVER HALIDE LAYER ON A SUPPORT, THE SILVER SALT IN SAID INNER LAYER BEING AT LEAST AS SOLUBLE IN AQUEOUS DEVELOPER SOLUTION AS THE SILVER SALT IN THE OUTER LAYER, (B) DEVELOPING SAID EXPOSED OUTER LAYER TO FORM A SILVER IMAGE IN SAID LAYER, BY A DEVELOPER COMPOSITION SUBSTANTALLY DEVOID OF SILVER DIFFUSION TRANSFER CHARACTERISTICS; (C) DEVELOPING THE OUTER LAYER WITH AN AQUEOUS DEVELOPER SOLUTION HAVING SILVER DIFFUSION CHARACTERISTICS CONTAINING: (I) A DEVELOPING AGENT TAKEN FROM THE GROUP CONSISTING OF A POLYHYDROXYBENZEND, P-AMINOPHENOL AND 2,4-DIAMINOPHENOL, (II) AT LEAST ONE DEVELOPING AGENT TAKEN FROM THE GROUP CONSISTING OF A POLYHDROXYBENZEND, PAMINOPHENOL, 2,4-DIAMINOPHENOL, P-METHYLAMINOPHENOL, 1-PHENYL-3-PYRAZOLIDONE, 1-PHENYL-4METHYL-3-PYAZOLIDONE, AND ASCORBIC ACID, AND (III) A SILVER HALLIDE SOLVENT, AND (D) REMOVING THE OUTER LAYER. 